In general, fiber reinforced plastic linear materials such as a glass fiber reinforced plastic linear material and the like are formed in a bar form or the like by impregnating reinforcing fiber such as glass fiber and the like with a resin, forcedly passing (introducing) it through a heated molding die, and pultruding it from the other end.
The linear material thus formed has excellent tensile strength because the reinforcing fiber is oriented in the lengthwise direction and is suitably used as a high tensile material. In particular, in the linear material as described above, a groove capable of accommodating an optical fiber can be formed continuously in the lengthwise direction on the outer surface thereof by appropriately modifying the cross-sectional shape of the molding die, the linear material has excellent tensile properties, and also the linear material can be formed in a long form. Therefore, the linear material is suitable as a tension member (tensile material) for optical communication cables.
However, since, as described above, the above linear material is formed by the continuous pultruding molding method, the groove accommodating the optical fiber is formed in parallel along the axial direction of the linear material. In a cable in which an optical fiber is placed in the groove provided in parallel along the axial direction, when the cable is bent, the optical fiber on the compression side is subject to compression force, and tensile force is applied to the optical fiber on the tensile side. Since the above compression force and tensile force increase the transmission loss of the optical fiber, it is preferred in order to avoid the above forces that the groove accommodating the optical fiber be twisted in a spiral in the axial direction so that the optical fiber placed in the groove is also twisted in a spiral. In accordance with the conventional continuous pultruding molding method for production of fiber reinforced plastic linear materials, since high tensile force is applied in the lengthwise direction of the linear material, it has been considered impossible to twist the grove in the axial direction of the linear material. For this reason, a tension member having a groove twisted in the spiral form relative to the axial direction of the linear material was formed a differently shaped rod of aluminum or an extruded product of a thermoplastic resin. A differently shaped rod of aluminum has a disadvantage in insulating properties and the thermoplastic resin extruded product has a disadvantage in heat resistance and strength. Thus, neither one performs satisfactorily as a tension member for optical communication cable.
The present invention has been made in view of the above circumstances, and its object is to provide a process for producing fiber reinforced plastic linear materials having high strength and excellent characteristics such as heat resistance and the like. In accordance with the present invention, fiber reinforced plastic linear materials having the groove twisted in the spiral form relative to the axial direction of the linear material can be obtained continuously.